Electroluminescent (EL) devices have been known for some years and have been recently used in commercial display devices and lighting devices. Such devices employ both active-matrix and passive-matrix control schemes and can employ a plurality of subpixels. In an active-matrix control scheme, each subpixel contains an EL emitter and a drive transistor for driving current through the EL emitter. In some embodiments, such as displays, the subpixels are located in an illumination area of the EL device, are arranged in two-dimensional arrays with a row and a column address for each subpixel, and have respective data values associated with the subpixels. Subpixels of different colors, such as red, green, blue and white, are grouped to form pixels. In other embodiments, such as lamps, EL subpixels are located in the illumination area of the EL device and are connected in series electrically to emit light together. EL subpixels can have any size, e.g. from 0.120 mm2 to 1.0 mm2. EL devices can be made from various emitter technologies, including coatable-inorganic light-emitting diode, quantum-dot, and organic light-emitting diode (OLED).
EL devices pass current through thin films of organic material to generate light. The color of light emitted and the efficiency of the energy conversion from current to light are determined by the composition of the organic thin-film material. Different organic materials emit different colors of light. However, as the device is used, the organic materials in the device age and become less efficient at emitting light. This reduces the lifetime of the device. The differing organic materials can age at different rates, causing differential color aging and a device whose white point varies as the device is used. In addition, each individual pixel can age at a rate different from other pixels, resulting in device nonuniformity.
The rate at which the materials age is related to the amount of current that passes through the device and, hence, the amount of light that has been emitted from the device. Various techniques to compensate for this aging effect have been described. However, many of these techniques require circuitry in the illumination area to measure the characteristics of each EL emitter. This can reduce the aperture ratio, the ratio of EL emitter area to support circuitry area, requiring increased current density to maintain luminance, and therefore reducing lifetime. Furthermore, these techniques require time-consuming measurements of representative devices before production to determine typical aging profiles.
Hente et al, in U.S. Patent Application Publication No. 2008/0210847, describe an OLED illumination device (a solid-state light or SSL), using one or more additional EL emitter(s) located outside the illumination area to serve as a reference against which to compare measurements of each subpixel. This scheme does not use the reference area during an illumination process (when the lights are on) so that the reference is always available to represent the initial, un-aged condition of the EL device. However, this scheme requires a fixed device characteristic which must be determined at manufacturing time. Furthermore, this scheme measures voltage or capacitance, so it cannot directly sense a change in light output due to a change in EL emitter efficiency, or a change in chromaticity of the light emitted by the EL emitter.
Cok et al., in U.S. Pat. No. 7,321,348, teach an EL display with a reference pixel outside the illumination area whose voltage is measured to determine aging. In this scheme, while the EL display is active (i.e. producing light for a viewer or user, such as when a light or television is turned on), the reference pixel is driven e.g. with an estimated average of the data values. In this way the reference pixel represents the performance of the display. Compensation is then performed for the whole display based on a measured voltage of the reference pixel. However, this scheme does not compensate for nonuniformity due to differential aging of adjacent subpixels, and does not compensate for chromaticity shift.
Naugler, Jr. et al., in U.S. Patent Application Publication No. 2008/0048951, teach a scheme for compensation which also relies on determining aging curves in the lab before production begins, and storing those aging curves in memory in each product. However, since this scheme uses curves taken before manufacturing, it cannot compensate for variations in those curves between individual panels, or for long-term shifts in the average characteristics of the displays manufactured due to aging of equipment, process changes, or material changes.
Cok et al., in U.S. Pat. No. 7,064,733, teach an EL display including one or more photosensors for detecting the output of subpixels in the illumination area. However, this scheme can reduce aperture ratio and reduce lifetime as described above.
There is a continuing need, therefore, for an improved method for compensating for aging of EL emitters in an EL device that can correct for differential aging, including chromaticity shifts, and for variations within and between manufacturing lots of EL devices, without reducing aperture ratio or lifetime, and without requiring extensive measurements before production begins.